Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due, in part, to the large volume occupied by a gas. Because the volume of an amount of gas is so much greater than the volume of the same number of gas molecules in a liquefied state, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and may not be economical.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline, jet fuel, kerosene, and diesel fuel have been decreasing and supplies are not expected to meet demand in the coming years. Fuels that are liquid under standard atmospheric conditions have the advantage that, in addition to their value, they can be transported more easily than natural gas, since they do not require liquefaction. Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures.
One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide is reacted with hydrogen to form organic molecules containing carbon and hydrogen. Those molecules containing only carbon and hydrogen are known as hydrocarbons. Those molecules containing oxygen in addition to carbon and hydrogen are known as oxygenates. Hydrocarbons having carbons linked in a straight chain are known as aliphatic hydrocarbons, which may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, which is used throughout the present specification). The molecules react to form hydrocarbons while confined on the surface of the catalyst. The hydrocarbon products then desorb from the catalyst and can be collected. H. Schulz (Applied Catalysis A: General 1999, 186, p 3) gives an overview of trends in Fischer-Tropsch catalysis.
The catalyst may be contacted with synthesis gas in a variety of reaction zones that may include one or more reactors. Common reactors include packed bed (also termed fixed bed) reactors, fluidized bed reactors, and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by fluidized bed reactors or gas-agitated slurry bed reactors. Fluidized bed reactors, which operate by fluidizing catalytic particles in a gas phase, are typically employed for high-temperature Fischer-Tropsch synthesis. Gas-agitated multiphase reactors sometimes called “slurry bed reactors”, “slurry phase reactor”, or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces gas bubbles. This type of slurry reactors is preferably used for low-temperature Fischer-Tropsch synthesis, when the reactor temperature is typically between 190° C. and 280° C.
As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques such as filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors, or slurry bubble column reactors, inherently have very high heat transfer rates, and therefore, reduced reactor cost. This, and the ability to remove and add catalyst online, are some of the principal advantages of such reactors as applied to the exothermic Fischer-Tropsch synthesis. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55) give a history of the development of various Fischer Tropsch reactors.
Typically the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of synthesis gas commonly contains a range of hydrocarbons including gases, liquids and waxes. It is highly desirable to maximize the production of high-value liquid hydrocarbons, such as hydrocarbons with at least 5 carbon atoms per hydrocarbon (C5+ hydrocarbons).
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Cobalt metal is particularly desirable in catalysts used in converting natural gas to hydrocarbons suitable for the production of diesel fuel. Further, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive, but the disadvantage of a high water-gas shift activity. Ruthenium has the advantage of high activity but is quite expensive.
Regardless of the particular metal being used, the catalyst is often supplied in a partially oxidized form. Thus, it is often desired to subject the Fischer-Tropsch catalyst to an initial reduction before the catalyst is used in a Fischer-Tropsch reactor so as to improve the initial selectivity and activity of the catalyst. The selectivity and activity of a typical Fischer-Tropsch catalyst will also, due to a number of factors, deteriorate over time. Thus, it is often desired to regenerate, or re-reduce, the catalyst periodically. The catalyst can be reduced/regenerated by exposure to a stream of hydrogen, or some other, gas.
It is often preferred to carry out the reduction process at an elevated temperature relative to the Fischer-Tropsch reactor temperature. At the preferred reduction temperatures, normally between 300 and 400° C., the reduction process may be susceptible to hydrogenolysis. Hydrogenolysis is generally defined as the breaking of a bond between two atoms in a molecule forming two product molecules, accompanied by hydrogen addition to each product molecule. More specifically, hydrogenolysis of hydrocarbons is defined as the breaking of a bond between two carbon atoms in a hydrocarbon molecule by reaction with hydrogen gas to give molecules with a reduced number of carbon atoms. Hydrogenolysis occurs in many chemical processes that produce useful products. G.C. Bond (“Catalysis by Metals”, Academic Press, New York, 1962) gives an overview and further details about hydrogenolysis. Typical Fischer-Tropsch catalytic metals such as cobalt, nickel, iron and ruthenium are also good hydrogenolysis catalysts. The breaking of the carbon-carbon bond over these metals typically occurs at a terminal carbon atom and results in the formation of methane. Whereas some processes take advantage of hydrogenolysis, it can be detrimental to a Fischer-Tropsch slurry.
Hydrogenolysis taking place in a Fischer-Tropsch slurry will tend to break down the heavy hydrocarbons in the slurry into lighter components and will also produce methane. This tends to lead to a loss of slurry liquid from the reaction vessel due to both methane formation and due to vaporization of the lighter components. The loss in liquid contributes to higher slurry viscosity, higher water concentrations in the slurry, and lower contact time between the reducing gas and the catalyst. The above factors can cause a loss of reduction efficiency of the metal oxide leading to lower syngas conversion rates during the Fischer-Tropsch process.
In order to minimize the effects of hydrogenolysis, Fischer-Tropsch catalyst reduction is conventionally performed either with the catalyst not in a slurry or at reduced temperatures that do not promote hydrogenolysis. Although initial reduction can be carried out before adding the catalyst to a slurry, it is not preferred because it adds additional processing steps and requires dedicated equipment. Similarly, separating the catalyst from a slurry for regeneration is difficult and also adds complexity and cost to the Fischer-Tropsch process. Carrying out regeneration or reduction at lower temperatures requires high amounts of noble metals in the catalyst, which adds to the cost of the catalyst.
Thus, there remains a need in the art for methods and apparatus to improve the efficiency and effectiveness of in situ catalyst reduction and/or regeneration processes. Therefore, the embodiments of the present invention are directed to methods and apparatus for constructing and operating a reduction system that seek to overcome these and other limitations of the prior art.